1. Field
An aspect of the invention relates to a multilayer printed wiring board that is applicable to a compact electronic apparatus, etc.
2. Description of Related Art
In association with a reduction in the weight and thickness of portable electronic equipment, greater packaging density and weight reduction are required even in connection with a printed wiring board used in the equipment. In a multilayer printed wiring board manufactured under a buildup method, the thickness of a copper plating layer forming a conductor layer of a buildup section (the thickness of a conductor foil film) is made smaller than the thickness of a copper plating layer of a core layer, to thus enable pursuit of lighter weight and a further slimming down.
However, when the thickness of the copper plating forming the conductor layer is reduced, an extremity of an open section penetrates into a conductor layer, to thus reach a lower layer, during boring operation using a laser beam for forming interstitial via holes, such as blind via hole or buried via holes. This raises a quality problem of formation of irregularly-shaped via holes whose bottoms are not filled with a conductor.
As a technique for forming via holes in a multilayer printed wiring board of this type, there have hitherto been present a technique (JP-A-2006-186029) for previously forming a barrier layer having the same thickness as that of a via hole pad and a metal-plated pad in bottoms of some via holes in such a way that all of the via holes become flat along a surface layer and a technique (JP-A-2002-198653) for enhancing connection reliability by forming open holes in the centers of positions of vias in an internal layer conductor circuit. However, these related-art techniques might not meet a requirement for a reduction in the thickness of a conductor layer in a buildup section while maintaining uniform quality of via holes.
As mentioned above, there has hitherto been a limitation in satisfying a requirement for a reduction in the thickness of a conductor layer in a buildup section while maintaining the quality of via holes constantly.